fix: Don't try to access undefined array key

Signed-off-by: Marcel Klehr <mklehr@gmx.net>

lib/private/TextProcessing/Manager.php

@@ -158,7 +158,11 @@ class Manager implements IManager {
 			throw new PreConditionNotMetException('No LanguageModel provider is installed that can handle this task');
 		}
 		$task->setStatus(OCPTask::STATUS_SCHEDULED);
-		[$provider,] = $this->getPreferredProviders($task);
+		$providers = $this->getPreferredProviders($task);
+		if (count($providers) === 0) {
+			throw new PreConditionNotMetException('No LanguageModel provider is installed that can handle this task');
+		}
+		[$provider,] = $providers;
 		if ($provider instanceof IProviderWithExpectedRuntime) {
 			$completionExpectedAt = new \DateTime('now');
 			$completionExpectedAt->add(new \DateInterval('PT'.$provider->getExpectedRuntime().'S'));